Sabrina Jahn and Jacob Klein, authors of “Lubrication of articular cartilage” (Physics Today, April 2018, page 48), write that “the fluid between the surfaces reduced the friction and supported much of the load via its own pressure.” That is, load on a joint pressurizes the fluid in the pores of the cartilages, providing self-pressurized hydrostatic lubrication. Loaded cartilage slowly loses pore fluid, but with each step the flexing and unloading of the joint and the bad fit of the cartilages to each other expose the surface of each to free fluid that it can resorb. I called this cycle “weeping lubrication” because cartilage weeps fluid when squeezed.

Experiments by Gerard Ateshian show that cartilage fully charged with pore fluid carries almost all its load by hydrostatic pressure.1 In the study discussed in reference 9 of Jahn and Klein’s article,2 a woman with an instrumented prosthetic hip rose from her chair, but little of the 20 MPa loading that Jahn and Klein mention would have been carried by high-spot to high-spot solid contact.

Because of support by hydrostatic pressure, joints carry high loads without overloading the boundary lubrication that synovial fluid provides, which, experiments show, fails at more than modest solid–solid loading.3 

I offer some advice for experimenters: When loaded cartilage eventually loses all pore pressure, the entire load transfers to the boundary-lubricated high spots and reveals their friction coefficient. For that to happen quickly, the sample must be very thin and on a very permeable mount.

1.
G. A.
Ateshian
,
J. Biomech.
42
,
1163
(
2009
).
2.
W. A.
Hodge
 et al,
Proc. Natl. Acad. Sci. USA
83
,
2879
(
1986
).
3.
C. W.
McCutchen
,
Wear
5
,
1
(
1962
);
C. W.
McCutchen
, in
The Joints and Synovial Fluid
, vol. 1, L. Sokoloff, ed.,
Academic Press
(
1978
), p.
437
;
T. A.
Schmidt
 et al,
Arthritis Rheum.
56
,
882
(
2007
).
4.
Sabrina
Jahn
,
Jacob
Klein
,
Physics Today
71
(
4
),
48
(
2018
).