Introduction: The accepted hypothesis for the maintenance of cartilage interstitial fluid load support under long-term joint loading is that contact migration leaves insufficient time for fluid exudation. However, it is also evident that the benefits of migration dissipate as the range of motion approaches first the contact length, a situation typical of moving diarthrodial joints, and then zero, a situation typical of static joints. This study quantifies the coupled effects of migration length, probe radius, contact area, normal force, and contact stress on cartilage fluid load support.
Methods: N=5 f19mm osteochondral plugs were extracted from the femoral condyles of mature bovine stifles and subjected to fluid load support testing in the migrating contact against glass spheres of varying size (Æ 6.4, 3.7, 2.4 mm) at varying loads (Fn = 5, 20, 100 mN), and with varying track lengths (0.5-7 mm) at Pe >> 1.
Results: Fluid load support depended primarily on the migration length per unit contact length (S*) and maintained a maximal magnitude (F*=100%) at S* > 10 – in other words, any track longer than 10x the contact length is effectively an infinite migrating contact. At S* < 10, the fluid load fraction varied as a sigmoidal function of S*, falling to F* = 50% by S* = 0.1 on average. This transition migration length, which corresponds to F* = 50%, was independent of probe radius and increased slightly but significantly with load (0.06 at 20 mN and 0.15 at 100 mN).
Discussion: When the migration length and contact length were of the same order of magnitude, the fluid load support of cartilage fell well below that predicted by the established mechanics of migrating contacts. We propose a simple analytical correction that should be used when S*<10. Although care must be used when extrapolating our results, the function describes all the data from this study (varying joints, samples, loads, contact areas, pressures) with R2 > 0.9768. These results have important clinical implications. First, given the relatively short tracks (S*~1) and high stresses in the joint (s ~0.5-5 MPa, respectively), they suggest that migration contributes less to fluid load support than expected based on the unmodified migration theory. Second, fluid retention and load support are impaired by reduced activity, reduced range of motion, and increased body weight.