Joints are not lubricated in the way Greene et al. say they are

If cartilage is exposed to fluid for a long time and is then loaded, Greene et al. (1) state, “Consistent with previous reports …, an initially low, but temporally increasing friction force characteristic of elastohydrodynamic lubrication (EHL) maintained by a fluid pressurization mechanism was observed. As the pore fluid is driven out and gradually becomes depleted in the increasingly deformed cartilage, the ability of the cartilage to support EHL via the secretion of interstitial fluid diminishes and a ‘mixed’ or ‘transition’ lubrication regime is entered involving a combination of EHL and boundary lubrication (BL) processes (i.e., EHL + BL). At longer loading times, the lubrication of the surfaces shifts more and more toward BL causing the friction force to rise until eventually plateauing at a stable equilibrium value (μ_eq ≈ 0.13) signifying steady-state sliding in a purely BL regime.”

The meaning of “driven out” of the cartilage, and of “via the secretion” is clear from their reference 14, which says, “Weeping lubrication is a non-contact hydrodynamic (HDL) lubrication mechanism important for determining the kinetic friction of joints governed by the pressure-driven flow of interstitial fluid” (2). Greene et al. (2) believe that hydrostatic pressure causes cartilage to expel fluid. This requires that the compressibility of cartilage solid exceed that of the fluid, and greatly exceed—most unlikely—if much is to come out. The Greene et al. (1, 2) mechanism is not weeping lubrication.

Unlike that of a hydrodynamic bearing, the friction of cartilage changes little with rubbing speed right down to zero. At zero speed a “squeeze film” of water, however thick, between rubbing surfaces 1 cm in radius and bearing a load of 100 kg, is thinned to 10,000 Å in 0.024 s by radially outward flow—shear-thinning synovial fluid would behave similarly—bringing the high spots on the cartilage surfaces into contact. But, as well as pressurizing the squeeze film, the joint load pressurizes the fluid in the cartilage pores. Once the contacts between the surfaces take enough of the load that the pressure in the squeeze film falls below that in the cartilage pores, fluid will flow from the cartilages into the film, greatly slowing its thinning (3). This is weeping lubrication (4).

The contacting high spots are at first pressed only lightly together, causing little friction, which rises slowly as the fluid in the cartilages becomes depleted. There is no progression to different modes of lubrication, just a steady handing-off of load from friction-free hydrostatic support to solid–solid contact.

The high spots are immersed in fluid at high pressure, but hydrostatic pressure has little effect on friction (4, 5).

Synovial fluid is a boundary lubricant for the high-spot contacts. Treated with hyaluronidase it pours like water yet lubricates about as well as before (4).

By its behavior, cartilage lubrication is neither noncontact nor hydrodynamic. Cartilage friction matches the predictions of weeping lubrication as described in ref. 4, not those of Greene et al.’s very different mechanism (1, 2) to which they once applied the same name (2).